Conventional cellular communication systems adopt a frequency reuse plan. Generally speaking, system antennas are erected at spaced apart locations. Each system antenna, along with transmitter power, receiver sensitivity, and geographical features, defines a cell. A cell is a geographical area on the surface of the earth within which communications may take place via a subscriber unit having predetermined operating characteristics and via the cell's antenna. In a cellular system that efficiently uses the spectrum allocated to it, system antennas are located to minimize overlap between their respective cells and to reduce gaps between the cells.
The spectrum allocated to a conventional cellular system is divided into a few discrete portions, typically frequency bands. Each cell is allocated only one of the discrete portions of the spectrum, and each cell is preferably surrounded by cells that use other discrete portions of the spectrum. Communications within a cell use only the discrete portion of the spectrum allocated to the cell, and interference between communications taking place in other nearby cells is minimized because communications in such nearby cells use different portions of the spectrum. Co-channel cells are cells that reuse the same discrete portion of spectrum. To minimize interference, the frequency reuse plan spaces co-channel cells a predetermined distance apart.
Communication systems almost always have a goal of efficiently using the electromagnetic spectrum allocated to them. In order to satisfy this goal, communication systems limit the opportunities for interference. Signals with significantly different frequency or timing parameters do not interfere and may easily be distinguished from one another. Likewise, a strong signal may be distinguished from a relatively weak signal having similar frequency and timing parameters. However, when generally equal strength signals having similar parameters are present, interference is possible. To reduce the likelihood of interference, a communication system often employs constraints which prevent the simultaneous presence of two substantially equal strength signals having substantially the same frequency within the system's area of coverage.
In conventional cellular systems, an area of coverage is divided into cells to efficiently use a given spectrum. Communication signals are intended to be transmitted and received within the confines of a single cell. Thus, transmission power levels are adjusted as low as possible while still insuring reliable reception within the cell. Adjacent cells are typically assigned different sections of the given spectrum so that no interference occurs between communications in adjacent cells. However, cells that are not adjacent to one another may reuse the same spectrum. Transmission power levels are sufficiently low so that no significant interference problem exists between communications taking place in non-adjacent cells.
With a satellite based cellular communication system sufficient cells can be provided to cover the entire surface of the earth. Such a system is described for example in U.S. Pat. No. 5,161,248, assigned to the assignee of the present invention, which is incorporated herein by reference.
Cellular communication systems are becoming more pervasive because they offer mobility, that is, the user may place and receive calls from anywhere in the service area and may generally move without restriction from one cell to another while using the system. However, this mobility also can create problems which do not arise in wired land-line systems. For example, users may unknowingly concentrate in a particular cell or small group of cells and cause a transient capacity overload in particular cells.
One solution to the problem of individual cell overload has been to have cells of different geographic sizes with each cell supporting a particular number of users. By making some cells smaller, users previously in those cells are now in adjacent, and hopefully, less heavily loaded cells. The change in cell size is typically accomplished by varying the power output from the transmitter for the cell, i.e., providing a lower power output for a smaller cell and a larger power output for a larger cell. This increases the total number of users that can be handled at the same time by spreading them more evenly over the available cells. Unfortunately there are limits to how small or large a cell can be made and still provide the needed communication link margin and/or avoid interfering with adjacent cells.
Another method used in the prior art to deal with individual cell overload is to temporarily assign some users or potential users who are located in the antenna overlap region near a cell periphery, from an overloaded cell into an adjacent (but overlapping) less heavily loaded cell. This method can provide some temporary relief by spreading a portion of the users to adjacent cells. No change in transmitter power is required since the overlap region exists under normal circumstances. However, the result is partly similar to the situation where the cell size is varied since those users near the periphery are shifted to the adjacent cells. This approach only provides relief in the border regions of the cell where there is overlap in the antenna coverage from immediately adjacent cells and, like the vary-the-cell-size approach, is only useful where immediately adjacent cells have unused spectral capacity.
A problem with overlap area reallocation is that the antenna patterns of adjacent cells must overlap to a significant extent in order for any significant number of users to fall within the overlap area. In general, this is detrimental to the capability and spectral capacity of each cell. The need to have antenna patterns extending greatly beyond nominal cell boundaries requires greater transmitter power and makes frequency reuse more difficult. Further, overlap reallocation may increase the number of hand-offs required during a call by a user.
The problems associated with uneven cellular loading have not been solved by spectral reuse plans. This is because the frequencies and time slot assignments to particular cells have generally been fixed at the time of construction of the system and no method has been available to vary the frequency or time slot assignments to particular cells as call loading varies.
Thus, what is needed is a cellular communication system that overcomes this limitation of the prior art by providing dynamic frequency/time slot assignment capability. Further, what is needed is the alteration in individual cell spectral capacity throughout the entire cell rather that just at the cell fringes as in prior art approaches using spectral reuse plans.
Thus, there continues to be a need for an improved method for varying the spectral capacity of individual cells in a cellular network in specific areas and at specific times to support additional users and to minimize the number of interrupted calls.